1. Field of the Invention
This invention relates to an apparatus and method for monitoring the concentration of an electroactive species in a flowing stream of solvent when the electroactive species of interest is present, not in a continuous fashion, but in a discrete high concentration zone or "slug." More particularly, the invention relates to the precise determination of an electroactive species of interest which is in a flowing sample stream which may include two types of contaminating species.
A contaminating species of the first type are large molecules. These are usually molecules in the so-called group of macromolecules. These species could be exemplified by polymer fragments in industrial fluids or blood proteins in biological fluids. These macromolecules are of such a chemistry that they may be absorbed to the surface of an electrode and thereby "poison" the electrode by placing it in an inactive state.
Contaminants of the second type are fundamentally different from contaminants of the first type. This second type of molecules includes small molecules, much the same size as the species of interest whose electrochemical activity we wish to follow. One common example of interfering species is as follows. It is common in the immobilized enzyme art to perform conversions of substrate molecules to hydrogen peroxide (M. W. 34) and measure it polarographically by the oxidation reaction: EQU H.sub.2 O.sub.2 .fwdarw.2H.sup.+ +1/2 O.sub.2 +2e.sup.-
Unfortunately, many biological samples which are of interest also contain significant amounts of uric (M. W. 168 in the keto form) and sometimes ascorbic acid (M. W. 176). Therefore, those systems which operate without a method to exclude these low mass (compared to the macromolecules which have masses on the order of thousands to hundreds of thousands) interfering molecules from the electrode do so with fairly high interference currents being generated. In some cases it is known that the current generated by the interfering species is at least as large as the current of the sample of interest.
Since polarographic methodology is based on additive currents, the species of interest signal may be distorted by the addition of these interference currents to the point where it has no analytical reliability.
2. Description of the Prior Art
In the past, electrode systems have been characterized by a few major types.
The general design of an electrochemical device was shown many years ago by electrodes like the one designed by Leland C. Clark, Jr., and shown in U.S. Pat. No. 2,913,386 entitled "Electrochemical Device For Chemical Analysis." In that system an electrolyte is maintained within a tube-like body electrode by a membrane whose primary function is to maintain the electrolyte with the electrode and to allow diffusable gasses to pass through the membrane.
These electrodes are designed for use in a static sample and have been called "dip-in" electrodes. In use, the electrode's tip is placed in the solution of interest, allowed to remain in the quiet, non-flowing solution until an accurate determination is completed.
This same dipping type electrode is shown in U.S. Pat. No. 3,380,805, also issued to Leland C. Clark, Jr., and entitled "Electrolytic Sensor With Anodic Depolarization." This patent discloses a trielectrode system with a membrane structure which performs much the same function as the membrane of the previously discussed Clark electrode.
These membranes were essentially total liquid barriers, and were not designed and did not function to allow electrolyte to pass the barrier. Rather, as suggested in Clark U. S. Pat. No. 2,913,386, the membranes were typically polyethylene which had the ability to allow gasses to diffuse therethrough, but in no case liquids.
Clearly, these electrodes were not to be used in flowing sample stream systems and they employed liquid nonpermeable membranes to separate solvent of the sample from the captive or internal reference electrolyte.
While the Clark electrodes were primarily for measurement of gasses, due to the nature of their membrane structures, they did allow interfering species of the same physical state as the sample of interest to interfere with the measurement. For example, if the electrode were being used to determine solution 0.sub.2 levels and there was present a substantial amount of CO or SO.sub.2, these interfering species could easily arrive at the electrode, just as the species of interest by permselective crossing of the membrane and generate an interfering current.
Unlike the Clark type electrode, many electrode combinations have been designed to attempt to measure species in a flowing steam.
U.S. Pat. No. 3,622,488, issued to Ramesh Chand entitled "Apparatus For Measuring Sulfur Dioxide Concentrations" shows a system to continuously monitor SO.sub.2 concentrations. Again, as in the Clark patents, a membrane is used to eliminate electrolyte loss yet allow diffusion of the SO.sub.2 across the membrane to the electrode's surface. While this type electrode does monitor the concentration of SO.sub.2 continuously it also has the drawback that interfering species will be allowed to reach the electrode and generate an interference current.
Many solution phase flow-through electrode systems have been demonstrated. In U.S. Pat. No. 3,707,455, issued to D. B. Derr et al., entitled "Measuring System" discloses a captive enzyme reagent. The reagent enzyme is trapped by a membrane. The membrane keeps the larger enzyme molecules inside a chamber and allows small molecules completely free diffusion across the membrane. Even though in a flowing stream, it is clear that small molecule interference in this system is still present, since a dual electrode system is used. One electrode measures species of interest plus interference and one only interfernece.
These systems are subject to the problems inherent in signal conditioning which affect signal reliability. Not only are electrodes like the ones discussed above subject to large molecule poisoning, but since large masses of unnecessary and interfering species arrive at the electrode and are reacted, there are electrodes are subject to more rapid degradation, concomitant failure, and drift. Also since these electrodes do measure large signals occasionally with small contributions from the species of interest, there is the problem of measuring a large volume of response with a small signal of interest and the associated signal-to-noise type problem.